Chemical vapor deposition (hereinafter CVD) methods are used broadly in the manufacture of semiconductor wafers. CVD involves exposing a semiconductor wafer to a reactive gas under carefully controlled conditions of elevated temperatures, sub-ambient pressures, and uniform reactant gas flow rate so that a thin, uniform layer of film is deposited on the surface of the substrate. There are several types of CVD processes, such as, for example, atmospheric CVD, low-pressure CVD, reduced-pressure CVD and plasma enhanced CVD.
A commonly used CVD system is a single-wafer CVD system utilizing a high-throughput CVD chamber. This is a result of the more stringent standards of film quality and increasing wafer sizes utilized in recent years.
For processing, typically, a CVD chamber is first heated to about 300.degree. C. to about 1,000.degree. C. A semiconductor substrate is mounted on a holding piece, called a susceptor, which is typically made of anodized aluminum. Then, processing gases are flowed into the chamber while the chamber pressure is regulated to an optimum level for achieving a uniform layer of film. The gases react to form a deposition layer on the surface of the wafer.
Since chamber pressure is an important processing parameter, means must be provided to accurately control such chamber pressure. The chamber pressure is controlled by regulating the flow rates of the gases and by an inlet flow regulating device, and by an exhaust flow control apparatus attached to the exhaust gas port of the vacuum deposition chamber. The exhaust flow control apparatus consists of a shut-off valve, a throttle valve and a vacuum pump with the shut-off valve typically connected directly to the exhaust gas port of the reaction chamber. During a standard deposition process, the shut-off valve remains open while the throttle valve opens and closes repeatedly to regulate the gas pressure in the chamber. The throttle valve is controlled by a servo-motor which is in turn controlled by a close-loop control system based on the feed-back signals from a pressure manometer mounted in the reaction chamber.
In a conventional deposition process, reactant gases enter the reaction chamber and produce films of various materials on the surface of a substrate for various purposes such as for dielectric layers, for insulation layers, etc. The various electronic materials deposited include epitaxial silicon, polysilicon, silicon nitride, silicon oxide, refractory metals such as titanium, tungsten and their silicides. In these film deposition processes, most of the material from the reactant gases is deposited on the substrate surface. However, it is inevitable that material is also deposited on other surfaces inside the chamber and on the throttle valve. This is especially true, in depositions of certain materials such as silicon oxide which requires a relatively high chamber pressure. It has been observed that the higher the deposition pressure required for a certain material, the more unwanted film is deposited inside the throttle valve.
In order to ensure the reproducibility of high quality wafer production, the reactant gas pressure inside the reaction chamber must be precisely controlled. This in turn requires the precise operation of the throttle valve in the exhaust flow control system. In a prior art CVD system, the exhaust flow control system consists of a shut-off valve installed immediately adjacent to the reaction chamber exhaust gas port, a throttle valve installed downstream from the shut-off valve at a distance of approximately 6-10 inches away from the reaction chamber exhaust port, and a vacuum pump installed downstream from and in fluid communication with the throttle valve and the shut-off valve. In this arrangement, semiconductor materials that are deposited at high chamber pressures, i.e., higher than 400 Torr, tend to deposit on the throttle valve after a certain number of deposition cycles.
In the normal operation of a CVD system, after each vacuum deposition process of coating a semiconductor substrate, a cleaning gas or a mixture of cleaning gases is purged through the reaction chamber in order to clean the chamber interior which includes the chamber walls and the susceptor. One such cleaning gas system is a mixture of nitrogen trifluoride, hexafluoroethane and oxygen for cleaning unwanted silicon oxide films from the chamber interior. Even though a plasma gas is ignited in the chamber to enhance its cleaning efficiency, the reactive species of the cleaning gas cannot reach the throttle valve for effective cleaning due to their limited lifetime. As a consequence, after several deposition processes are conducted in the reaction chamber, i.e., between 500 to 1,000 cycles, a sufficient amount of unwanted silicon oxide film is deposited and remains on the throttle valve to render it unfunctional, that is, the deposited material increases the diameter of the valve causing the throttle valve to scrape against the valve chamber as the throttle opens and closes. The scraping action prevents a smooth motion of the throttle valve; instead, the throttle valve functions sluggishly and is no longer capable of providing accurate pressure control in the reaction chamber. This basically inoperable throttle valve leads to poor pressure control in the reaction chamber and thereby contributes to the production of silicon wafers having poor repeatability and reliability.
Periodically, after approximately 500 to 1,000 deposition cycles, therefore, the throttle valve assembly must be completely disassembled and manually cleaned by a wet chemistry technique. This is a very laborious and time consuming process which leads to poor throughput and increased cost of manufacturing. Furthermore, after each manual cleaning process, the entire exhaust flow control system must be recalibrated in order to resume the production of silicon wafers in the reaction chamber.
It is therefore an object of the present invention to provide an in-situ cleaning method for the throttle valve used in a vacuum deposition device that does not have the shortcomings of the prior art methods.
It is another object of the present invention to provide an improved in-situ cleaning method of a throttle valve without extensive equipment modifications.
It is a further object of the present invention to provide an improved in-situ cleaning method for a throttle valve used in a vacuum deposition device which can be effectively carried out in the regular chamber cleaning process after each deposition cycle.
It is yet another object of the present invention to provide an exhaust flow control system that includes a throttle valve for regulating the gas pressure in the vacuum deposition chamber which can be cleaned in-situ with the chamber interior during a regular chamber cleaning cycle.